Well, yeah ...
Ya said the car has no a/c and she is in traffic a lot...so...
I would simply put a cabin ON/OFF sw somewhere convienent and run the wires to the top of the a/c drier switch....when she wants fan, she throws the switch..or you can just get a radiator sensor switch to do it auto...
Those fans push a lot of air and we used to always put a cabim sw on those chassis....just cuz it was simple and nice to have on a hot day , or towing , or traffic ,or whatever...cost $2-3 to do